THE JOURNAL OF
Organic Chemistry 0 Cofiyrighl 1958 by ths American Chemical Society
Volume 22, Number 12
January 9, 1958
[CONTRIBUTION FROM THE CHEMICAL LABORATORIES O F THE UNIVERSITY O F NOTREDAME]
Dehydration of cis- and trans-2-Phenylcyclohexanols' ERNEST L. ELIEL, JOSEPH W. M c C O Y , ~AND ~ CHARLES C. PRICE2b Received April 22, 1967
The dehydration of trans-2-phenylcyclohexanol by 85% phosphoric acid has been reinvestigated using infrared, ultraviolet, and mass spectrometry to ascertain the composition of the products. Extensive rearrangement occurs to give a product containing ca. 14y0 1-phenylcyclohexene, 20% unconjugated 3- and 4phenylcyclohexenes, 17% benzalcyclopentane,46y0 unconjugated 1-, 3-, and Pbenzylcyclopentene and 4% 1,3-endoethylene-l,2,3,4tetrahydronaphthalene.These findings are in substantial agreement with results independently obtained by Schaeffer and Collins,s as is the observation that dehydration of cis-2-phenylcyclohexanol involves substantially no rearrangement. Pyrolysis of 2-phenylcyclohexyl methyl sulfite4 led to 57% 1-phenylcyclohexene and 43% 3-phenylcyclohexene with less than 2% ring contraction product starting with the cis isomer. On the other hand, in the case of the trans isomer about 21% of the pyrolysis product had suffered ring contraction.
Introduction. It was reported earlier,b on the basis of refractive index measurements, that in the dehydration of the 2-phenylcyclohexanols by 85% phosphoric acid, the cis isomer (I) gave largely 1phenylcyclohexene, whereas the trans isomer (11) led to 3-phenylcyclohexene. The subsequent preparation, by other means, of pure 3-phenylcyclohex-
Q
HsCLj OR
I, R=H 111, R=OSOZCHs
-
11, R=H
IV, R=OSO&Ha
ene and measurement of its refractive however, invalidated the earlier conclusion with respect to the product composition from the trans isomer (11).We have therefore reinvestigated the products of the trans isomer (11) by analysis with the now readily available tools of ultraviolet and infrared spectroscopy and mass spectrometry. At the same time we reinvestigated briefly the products of dehydration of the cis isomer (I) as well as those of the pyrolysis of the 2-phenylcyclohexyl methyl sulfites, I11 and IV since these products had also been deemed to be mixtures of 1- and 3-phenylcyclohexene on the basis of refractive index 0nly.4~6 Our results confirm (and, in one minor respect, extend) the findings of Schaeffer and Collins3-obtained by the entirely different technique of isotope dilution- that dehydration of trans-2-phenylcyclohexanol (11) (but not of the cis isomer I) leads to extensive ring contraction. Ring contraction, though to a lesser extent, also occurred in the pyrolysis Of the trans methyl sulfite IV but not with cis-2-phenylcyclohexyl methyl sulfite (111). Results. Dehydration of trans-2-phenylcyclohexanal with ss70 phosphoric gave a hydrocarbon fraction whose refractive index (1.5493-
(1) Presented before the Division of Organic Chemistry, AMERICANCHEMICAL SOCIETY, MinneaPOk Minn., SePtember 15, 1955. Contribution from the Radiation Project of the University of Notre Dame supported in part under Atomic Energy Commission contract AT( 11-1)-38 and Navy equipment loan contract Nonr-06900. (2) (a) -From the Ph.D. thesis of J. W. McCoy. (b) Present address: Department of Chemistry, University of Pennsylvania, Philadelphia 4, Pa. (3) H. J . Schaeffer and C. J. Collins, J . Am. Chem. SOC., 78, 124 (1956). (4) G. Herti, J. Am. Chem. Soc., 76, 1213 (1954). (6) E. R. Alexander and A. Mudrak, J. Am. Chem. Soc., (5) C . C. Price and J. V. Karabinos, J. Am. Chem. Soc., 72, 1810 (1950). 62, 1159 (1940). (7) A. Berlande, Compt. rend., 213, 437 (1941). 1533
1534
ELIEL, MCCOY, AND PRICE
VOL.
22
TABLE I PROPERTIES OF REFERENCE HYDROCARBONS Compound
Ref.
1-Phenylcyclohexene
124-127 128 82
3-Phen ylcy clohexene
235 7&79
Phenylcyclohexane
Benzalcyclopentane l-Benzylcyclopentene
14-15 16 1
atm. 2
118-119
19
1.5440(26') 1.5448 1.5448 1.5444 1.5417 (25") 1.5449
105-115 11-113 107
10-15 13 13
1.5249 1.5237 (25') 1.52556
108-118 86
8 1.9
(1. 5518)d 1.5752
113-117 103-107 108-109
18 11 14
1.5355-1. 5510e 1.5367 1.5363
70-72 103-104
5 12.5
1,5170-1.5245, 1.5200 1.517@
117 115 108-108.5
25 15 13.5
Benzylcyclopentane
1,3-Endoethylene1,2,3,4-tetrahydronaphthalene (V)
1.5692 1.5645 (25") 1.5660 (25") 1.5690 1.5664 (25") 1.5692
1.55550
247 246 248 247 I
253 248 248
I
248
248
248
I
12,OOOU I
12,120 12, 170b 12,940 -
__ I
653 232 7750 -
.17,150' I
750b
__ -
200b
9 10 11 12 13 14
7 6
3 14 13 e
9 14 c
3 c
9 3
c
9 15 c
16 17 C
In cyclohexane. Data used in calculations, see text. This work. Only 66% purity claimed for this sample. e At temperatures ranging from 17.5 to 23". 'At temperatures ranging from 17 to 21".
1.5498) was considerably different from that (1.5553) previously observed.6:8Estimation of the composition of this materiai-assuming it to be a binary mixture of 1- and 3-phenylcyclohexane-on the basis of refractive index and ultraviolet extinction coefficient at 252 mp (cf. Table I) led to inconsistent results. Moreover, the infrared spec(8) Calculated from the refractive indices of hydrocarbon standarde and. the composition of the products reported in ref. 5. (9) G. Egloff, "Physical Constants of Hydrocarbons," Reinhold Publishinn CorD.. New York, N. Y., Vol. 111 (1946), pp. 265, 276. (101 A. @. Cone. F. S. Fawcett. and G. Munn. J. Am. Chbn.' SOC.,72, 3'399 (1950). (11) R. C . Carlin and H. P. Landerl, J . Am. Chem. SOC., 7.5,3969 (1953). (12) R. T. Arnold and P. N. Richardson, J . Am. Chem. SOC.,76, 3649 (1954). (13) J . Weinstock and F. G . Bordwell, J . Am. Chem. Soc., 77, 6706 (1955). (14) R. Y . Mixer and W. 6. Young, -. J . Am. Chem. SOC. 78;3379 (1956.). (151 D. V. Niehtineale and M. Maienthal, J. Ani. Chem. So;., 72, 4823 (1550).(16) W. Baker and W. G . Leeds, J. Chem. Soc., 974 (1948). (17) L. H. Groves and G. A. Swan, J. Chem. SOC.,871 (1951). _
I
trum of the dehydration product showed numerous bands not present in either 1- or 3-phenylcyclohexene. On the assumption that part of the material might have suffered ring contraction (cf. ref. 15), it was hydrogenated and the infrared spectrum of the hydrogenated material was recorded and compared with the spectra of pure phenylcyclohexane and benzylcyclopentane. Both hydrocarbons were clearly present. Mass spectrometric analysis of the hydrogenated material indicated it to be ea. 35% phenylcyclohexane and 65% benzylcyclopentane. Moreover, the presence of benzalcyclopentane in the dehydration product prior to hydrogenation was suggested by the isolation of benzaldehyde from an ozonolysis reaction. Careful inspection of the mass spectrum of the hydrogenated product disclosed a very large residual peak a t mass 129 not present in the mass spectrum of either phenylcyclohexane or benzylcyclopentane. This peak also occurred in the spectrum of the product before hydrogenation. Since 1,3endoethylene-1,2,3,4-tetrahydronaphthalene(V) is reported to be obtained in the cyclization of the olefin resulting from dehydration of l-benzylcyclopentanol,17 and since this hydrocarbon might well have a large mass peak a t mass 129, due to loss of
DECEMBER
1957
1535
DEHYDRATION O F 2-PHENYLCYCLOHEXANOLS
TABLE I1
This work: Ref. 3
20% 21%
17%
13% 15%"
Ca. 2094
a 9% of 3-phenylcyclohexene and 6% of 4-phenylcyclohexene. analyzed for.
v the ethylene bridge subsequent to ionization in the mass spectrometer chamber, the presence of V in the dehydration product of I1 was suspected, especially since phthalic anhydride (in small yield) was isolated from the permanganate oxidation of the product mixture obtained in the dehydration of PI. The mass spectrum of a synthetic sample of compound V16 does, indeed, have an unusually intense peak at mass 129 and mass spectral analysis of the mixture obtained in the dehydration of I1 followed by hydrogenation showed it to consist of 33.5% phenylcyclohexane, 62.6% benzylcyclopentane, and 3.9% V. Assuming this composition there were still small residual peaks a t mass 41 and 65. It is possible that these may be due to very small quantities of other isomers, such as 3,4-benzo[3.3.0]bicyclooctane (VI) but this possibility was not explored further.
VI
I n any case, a synthetic mixture of the above composition (33.5: 62.6 : 3.9) agreed well with the hydrogenation product in refractive index (1.5213 vs. 1.5217) and infrared spectrum. Complete calculation of the composition of the dehydration mixture of I1 from ultraviolet and refractive index data was unfortunately not feasible, since the data were not sufficiently mathematically independent to allow a sensitive calculation. The approximate benzalcyclopentane content of the mixture was therefore estimated as 17% by comparison of the 1 1 . 6 1 infrared ~ band of the product mixture with that of synthetic mixtures of varying benzalcyclopentane content. It follows that the benzylcyclopentene content of the mixture must be 46%, since the total benzylcyclopentane after reduction amounts to 63% (vide supra). The distribution of the phenylcyclohexene fraction as between conjugated and unconjugated isomer can then be calculated from the ultraviolet data (Table I, marked values) to be 20% 1-phenylcyclohexene and 13% 3-phenylcyclohexene.1s (18) The infrared spectrum of the product closely resembled that of a synthetic mixture containing 20%
I-phenylcyclohexene, 13% 3-phenylcyclohexene, 17'% benzalcyclopentane, 46% I-bcnzylcyclopentene, and 4% V.
46 %
4%
(32%)b
This figure refers to l-benzyk!yctopentcne only.
Not
It is of interest to compare out data with those of Schaeffer and C ~ l l i n sSince . ~ these investigators have shown that 4-phenylcyc!ohexene as well as 3phenylcyclohexene is present, in the dehydration mixture of 11, and since neither of these isomers would be expected to have strong ultraviolet absorption, it is evident that o ~ purported r analysis of 3-phenylcyclohexene (based on UV data) is, in fact, an analysis of unconjugated (combined 3- and 4-)phenylcyclohexene. Similo rly, our analysis of benzylcyclopentene (which is kmed on the difference between total material giving benzylcyclopentane on hydrogenation arid henzalcyclopentane as analyzed by infrared) is, h fact, an analysis for total unconjugated (1-, 3-, aiid 4)benzylcyclopentene.Ig Taking these facts into account our analysis and that of Schaeffer and Collins? are summarized in Table 11. It is evident that the two sets of data are in good agreement, except in the case of benzylcyclopentene. However, in this case our figure, as explained above, refers to total benzylcyclopentene whereas that of Schaeffer and Galiins3 refers to 1berizylcyclopentene only. Sirice the data of Schaeffer and Collins account for only about 88% of the distilled material, and of the missing S2%, only 476 are accounted for by V, the discrepancy would disappear if the mixture contained about 11 =t 3% of 3- and 4-benzylcyclopentene. In any case, our maSb spectral analysis shows convincingly that the dehydration mixture contains 6370 of material rcducible to benzylcyclopentane. Dehydration of cis-2-phecylcyclohexanol (I) with 85% phosphoric acid gave a hydrocarbo!: mixture whose refractive index (1.563741) was in good agreement with earlier findings (1.5644558). Hydrogenation of this product led to material which, according to mass spectrometric analysis, contained 97.4% phenylcyclohexane and 2.67; benzylcyclopentane, indicating that very little ring contraction had occurred in the dehydratiorL of I. Schaeffer and Collins3 rcport that the dehydration product contains 88% l-pheiiylcyclohexene and 2% 3-phenylcyclohexene leaving open the question of the remaining 10% of rnatesial. It is
(19) There is an implicit assumption here that 3- and 4benzylcyclopentene do not absorb appreciably a t 11.61~, the band used for analyzing benzalcyclopentanc. The genera! consistency of our resnlts, both internally and with tfiosc u l ref. 3 suggests that this assumption is sound.
1536
ELIEL, MCCOY, AND PRICE
evident that not all this material is a product of ring contraction. The finding of ring contraction in the dehydration of trans-2-phenylcyclohexanol (11) suggested a reexamination of the pyrolysis of cis- and trans2-phenylcyclohexyl methyl sulfite (I11 and IV),4 since the possibility of ring contraction was not previously considered in the analysis of the products of this reaction. Pyrrolysis of the cis isomer (111) gave a hydrocarbon fraction boiling at 115-130"/ 15 mm. The refractive index of the product (1.5583) was substantially lower than that (1.5625) reported earlier.4 The discrepancy may be due to the fact that in the previous investigation4 the reaction product was distilled over sodium a t atmospheric pressure, since we were able to show (see Experimental) that 3-phenylcyclohexene, one of the components of the pyrolysis mixture, is not stable under such conditions.20 Hydrogenation of the pyrolysis product of I11 gave material which, according to mass spectrometric analysis contained 98.3% phenylcyclohexane and 1.7% benzylcyclopentane, proving substantial absence of ring contraction. The pyrolysis product, according to refractive index and ultraviolet absorption = 7,140) contained 57% 1-phenylcyclohexene and 43% 3-phenylcyclohexene. (Calculated for this composition: ng 1.5585, 8252 7,140; assuming ng 1.5692, t262 12,000 for 1phenylcyclohexene, ng 1.5444, E262 700 for 3-phenylcyclohexene.) This finding changes the previous data only qualitatively . In contrast, hydrogenation of the pyrolysis product of trans-2-phenylcyclohexyl methyl sulfite IV gave a product which, according to mass spectrometric analysis, was 78.6% phenylcyclohexane and 21.4% benzylcyclopentane. Here, evidently, ring contraction had taken place (though to a lesser extent than in the phosphoric acid dehydration of alcohol 11) and the previous analysis of the pyrolysis product4 is therefore not valid. A complete analysis of this product was not attempted in the present investigation. Synthesis of reference hydrocarbons. 1-Phenylcyclohexene, 3-phenylcyclohexene, and l-benzylcyclopentene were prepared by methods described in the literature,6JJ1 and the properties of two of the reaction products are shown in Table I to be in good agreement with those reported by other investigators. considerable difficulties were encountered in the synthesis of benzalcyclopentane. The (20) Berti's crude product had about the same refractive index (1.55804) as our product. Similarly, our pyrolysis product from the trans-sulfite IV had n y 1.5550, in fair agreement with Rerti's product prior. to distillation (ny 1.5560) but in poor agreement with his distilled product ( n z 1.5698). H. Pines and M. Kolobielski, J. Am. Chem. Soc., 79, Iii98 (1957) have recently shown that I-phenylcyclohexene undergoes disproportionation and dehydrogenation when heated above 200' with sodium-benzylsodium. (21) Y . I. Benisenko, Ber., 69, 1668 (1936); L. Piaux, Compt. rend., 199, 1127 (1934).
VOL.
22
best sample of benzalcyclopentane obtained in this investigation resulted from the dehalogenation of phenyl(1-chlorocyclopenty1)bromoethane (VII) with zinc; this sample was analytically pure and had n? 1.5752; 8252 18,400. Successive fractions of this material did not differ in boiling point or refractive index. Slightly impure benaalcyclopentane (n? 1.5655) was obtained by the Chugaev reaction of cyclopentyl phenyl carbinol (VIII) ; this material appeared to contain some sulfur-containing byproducts but its infrared spectrum was very similar to that of the material described above. Other likely procedures failed to yield benzalcyclopentane. 22 Dehydration of 1-benzylcyclopentanol (IX) with iodine and with oxalic acid gave the unconjugated isomer 1-benzylcyclopentene, which was also the major product of the dehydrohalogenation of 1-benzylcyclopentyl chloride (X) with trietha-
Br
VI1
VI11
nolamine. This preference for formation of the nonconjugated isomer is surprising. It must be due to the preference for an endocyclic bond over an exocyclic one,2a since dehydration of benzyl diethyl carbinol, CJ&CHZCOH(CZH~)Z and dehydrohalogenation of the corresponding chloride, CeHsCH2CC1(C2H&, give the conjugated olefin l-phenyl-2ethyl-Zbutene, CeH&H=C (CZHE) 2. 24 Other preparations of benzalcyclopentane which failed are dehydrohalogenation of cyclopentylphenylchloromethane, CeH6CHClC5Hewith boiling pyridine (quaternization appeared to occur), and with boiling aqueous triethanolamine [cyclopentyl phenyl carbinol (VIII) resulted] ; dehydration of cyclopentyl phenyl carbinol (VIII) with potassium hydrogen sulfate or phosphoric acid (complex mixtures resulted) : and dehydration of l-benzylcyclopentanol IX with hot solid potassium hydroxide. The latter procedure yielded toluene in almost mole-per-mole yield, apparently as the result of a base-induced cleavage. The other product of the assumed cleavage, cyclopentanone, was probaly' resinified by the potassium hydroxide. It is known that treatment of benzyl dimethyl carbinol, (22) Difficulties in the preparation of benzalcyclopentane are also reported in ref. 3. (23) Cj. H. C. Brown, J . Org. Chem., 22, 439 (1957). (24) A. Klages, Ber., 37, 1724 (1904); K. Auwers and F. Eisenlohr, J. prakl. Chem., [a] 82, 94 (1910). Unfortunately the structural evidence for the product is not convincing. See also J. M. Lamberti and P. H. Wise, J . Am. Chem. Soe., 75, 4787 (1953).
DECEMBER
1957
DEHYDRATION O F
2-PHENYLCYCLOHEXANOLS
1537
TABLE I11 E L I M I N A T I O N FROM 2-PHENYL-SURSTITUTED
x in
J(';
OH OH OAc OAc OCSCHs OCSzCH3 0S02CH8
OSOzCHa 2"
N+(CII3)3 N+(CIIs)3 0 + N(CH3)2 0 N(CH3h a
Stereoisomer
Method
1-Phenyl"
Hap04
8g3 17; 213
Cis
trans Cis trans cis trans cis
trans trans cis trans Cis
trans
Pyrol. Pyrol. Pyrol. Pyrol. Pyrol. Pyrol.
HNOi OH, A OH, A Pyrol. Pyrol.
7 86.5 0-4 88 57 d
4 100 100 9
85
CYCLOHEXYL COMPOUNDS Products % 3-Phenyla
Ring Contr.
23 9C 93 13.5 96-100 12 43
36
63b 0 0 0 0
